1. Field of the Invention
The present invention is directed generally to the control of vibration in structures, and more particularly to the confinement of vibrational energy to selected portions of structures.
2. Description of Related Art
The suppression or control of vibration has an increasing importance in the design, manufacture, operation, maintenance, precision, and safety of structures and machinery. Engineering systems are subjected to numerous disturbances from either internal or external sources of vibration. Conventional methods for reducing the effect of vibration take several forms, and may be classified into the three general categories, viz. 1) isolation, e.g. the use of rubber shock mounts, 2) absorption (redirection), and 3) suppression (dissipation).
Conventional active vibration control methods utilize sensors, signal processing, actuators, and power sources to produce forces or strains in the system that counteract the vibration or to effectively increase the dissipation in a system.
"Smart" materials and structures have extended the range of active, as well as passive, vibration control mechanisms, where the term "smart" refers to materials or structures that respond to environmental or operational conditions by altering their material, geometric, or operational properties. Such a response may be triggered both with and without additional control mechanisms (such as a sensory and feed-back loop). Examples of smart materials include piezoceramics, shape memory alloys, electrostrictive and magnetostrictive materials, rheological and magnetological fluids.
Although active control methods have been shown to be effective in some limited applications, their drawbacks are emphasized by a reliance on computationally complex control algorithms, high numbers of sensors and high actuator power requirements, and continuous monitoring and feedback or feed-forward mechanisms. These drawbacks have demonstrated the need for an alternative or additional approach to vibration control. Additionally, semi-active control techniques reduce only the requirement on continuous actuation but their development and implementation has not yet progressed as far as fully active control or passive control.
It is important for the economic operation and practical implementation of active and passive vibration control technologies that the number of controlled regions and controlling components be reduced to achieve the vibration control objectives more effectively and efficiently.
There are common features between the above methods. First, they are designed to control vibrations in a reactive manner. All of these methods assume (or necessitate) that excessive vibration energy is present in all regions of a structure which are to be controlled. The vibration control mechanism then acts upon this vibration energy to suppress vibration. Second, these methods are all designed to be most effective in a certain frequency range. Isolators, absorbers, and dampers, whether active or passive, must be tuned to a specific frequency range of interest. Active cancellation methods are also limited in their effective frequency range by the speed of signal processing and activator response time requirements. Third, these methods are designed without regard to the distribution of vibrational energy throughout the system.
Therefore, there is a need for a method of controlling vibrational energy in a system which is proactively designed into the system, and which takes account of total energy distribution throughout the system. There is also a need to expand the frequency range over which vibrational energy is controlled. Further, economic considerations drive a need to reduce the number of controlled regions and controlling components and to reduce the complexity of active vibration control systems.